lint: make char unsigned on aarch64

This fixes tests msg_074 and msg_076, which previously warned for
'\xff': conversion of 'int' to 'char' is out of range [119]

The commit from 2014-08-10 that first defined char as signed had the
remark "Enough for a distribution build".  At that time, there was no
unit test for lint1 that would have detected the signedness of char.

(rillig)